Please refer to FIG. 1, which illustrates a servo control system of an optical disc drive, in which, the optical disc drive comprises an optical pickup head (“PUH”) 10, and a disc 110 including a center hole can be fixed on a turn table 122 and be rotated by a spindle motor 120. The driver 180 can generate driving control forces to enable the optical pickup head 10 to produce a radial-direction movement and a focusing-direction movement.
The weak signals that are generated when optical pickup head 10 is accessing disc 110 are received and processed by pre-amplifier 150 and a servo error signal set is outputted by the pre-amplifier 150. The servo error signal set, for instance, includes a radio-frequency (RF) signal, a sub-beam added signal (SBAD) signal, a wobble signal, a tracking error signal (TE) and a focusing error signal (FE), etc. The servo error signal set is input into a controller 170 for process and application.
According to the servo error signal set, the controller 170 can output a servo control signal set to the driver 180. The servo control signal set includes at least a tracking control signal and a focusing control signal; and a driver 180 comprises at a least tracking coil and a focusing coil. When the tracking control signal is input into the tracking coil, a driving control force is generated to enable the optical pickup head 10 to produce a radial-direction movement. Also, when the focusing control signal is input into the focusing coil, another driving control force is generated to enable the optical pickup head 10 to produce a focusing-direction movement. Therefore, under normal operation, the driving control forces can maintain the optical pickup head 10 on appropriate focusing position and move along the disc track.
However, because imperfections in the disc manufacture process or scrape of disc both result in defects of disc, when optical pickup head 10 meets a defective region as it reads the disc, the servo error signal set output from the pre-amplifier 150 will be abnormal, which leads to the output of abnormal servo control signal set by controller 170. This further causes inability of driver 180 to control optical pickup head 10 correctly and optical disc drive servo control system becomes unstable. When the servo control system is unstable, the optical pickup head 10 may produce either focusing failure or off track which leads to the disc not able to be accessed or access error. Furthermore, the controller 170 comprises an auto gain controller (AGC) in order to adjust dynamically gain of the wobble signal. When optical pickup head 10 meets a defective region as it reads a disc, auto gain controller will continuously increase the gain of wobble signal because the wobble signal is too small.
The US publication number 20050265170 provides a method for determining defective blocks of optical disc. The method compares sub-beam added signal (SBAD) and its low-pass filtered signal (SBADlowpass). When the absolute value of the difference between the two signals is greater than a threshold, the related region will be defined as a defective region. Please refer to FIG. 2, which illustrates the signals of conventional optical disc drive detecting defective region of disc. As the optical pickup head emits a beam along the track for accessing data, if the track appears a bright defect, like a region with too strong reflecting rate, sub-beam added signal (SBAD) will rise sharply and its low-pass filtered signal (SBADlowpass) will rise slowly. When the absolute value of the difference between the two signals exceeds a value M, a defective signal (DEFECT) generated by the controller 170 will change to a first level (such as high level). When the absolute value of the difference between the two signals becomes lower than the value M, defective signal (DEFECT) will return to a second level (such as low level).
By the same logic, if the track appears a dark defect, for instance the scrape region, sub-beam added signal (SBAD) will drop sharply and its low-pass filtered signal (SBADlowpass) will drop slowly. When the absolute value of the difference between the two signals exceeds a value M, the defective signal (DEFECT) generated by the controller 170 will go to the first level. When the absolute value of the difference between the two signals becomes lower than the value M, the defective signal (DEFECT) will return to the second level. By using the defective signal (DEFECT), the driver 180 can adjust driving control forces in time for optical drive to avoid focusing failure or off track.
Further, radio frequency (RF) signal can also be used to determine the defective region of a disc. Please refer to FIG. 3, which illustrates signal diagram of conventional optical disc drive detecting defective region. The method diagnoses the high-pass filtered radio frequency signal (RFHigh—Pass). As optical pickup head emits a beam along the track to access data, if the beam meets a defective region, the high-pass filtered radio frequency signal (RFHigh—Pass) will suddenly drop, pre-amplifier 150 will then change the defective signal (DEFECT) to a first level (e.g. high level) and not return to a second level until RFHigh—Pass returns to normality. By using the defective signal, driver 180 can adjust driving control forces in time for optical drive to avoid focusing failure or track off.
The aforementioned methods determine a defective region by using energy summation of reflecting laser beam to generate RF signal or SBAD signal. However, as track shape becomes defective during manufacture process, energy summation of reflecting laser beam does not vary obviously. It results in drive cannot detect disc shape changed and cannot control optical pickup head. It also leads to focusing fail or track off so that data stored in the disc cannot be accessed.